A liquid crystal display (LCD) has limited operating temperature range due to the characteristics of liquid crystal (LC). As the temperature decreases, LC response time increases dramatically because of the increased viscosity. Consequently, a LCD cannot operate properly at low temperatures. Three approaches or heating systems for maintaining proper display operating temperature have been employed in existing LCD technologies. These heating systems include: (i) an external heater attached to the Indium Tin Oxide (ITO) glass surrounding an active pixel array, (ii) an internal row line heater, and (iii) an internal common electrode line (VCOM) heater. Each type of heating system has various shortcomings.
Typically, the external heater is attached to the Thin Film Transistor/Indium Tin Oxide (TFT/ITO) cover glass following display fabrication. Such an approach provides maintenance heating by conducting current through the LCD cover glass. Because the external heater provides heat along the edges of the LCD and through the cover glass(es), an external heater is generally inefficient and unable to rapidly warm the display during cold start conditions. The row line heater and the VCOM heater provide much higher efficiency and uniform heat close to the LC material and can be used during cold start conditions.
FIG. 1 shows a diagram of a standard pixel array 100 with polysilicon row lines 110 and polysilicon VCOM lines 130. The row lines 110 carry signals to control pixel transistors. CLC is the capacitance between the pixel electrode and the ITO common plate with the liquid crystal in between. VCOM serves as one plate of the pixel storage capacitors (Cstg) and is tied to a DC voltage during display operation. The DC voltage is effectively shielded from the LC by the active layer which resides between the VCOM lines and LC material. The active layer also serves as another plate of the storage capacitor.
The second approach is a row line heater which is an internal heater integrated into the active matrix architecture of the display. The row line heater can be used during cold start conditions to rapidly warm the LC material. The row line heater is located within the pixel array, very close to the LC, so that it can provide high efficiency, uniform heating inside of the LCD glass.
FIG. 2 shows a row line heating scheme. Row line drivers 230, 232 drive current through polysilicon row lines 210 from one end to the other and supply heat to the pixel array 200. Since the row lines 210 also control pixel 220 operation, a display cannot operate during row line heating. The warm-up time needed during a cold start limits their usage in various applications of this heating scheme.
FIG. 3 shows a VCOM heating scheme. The VCOM heater utilizes the polysilicon VCOM lines 344 of the pixel array 300 as resistive heating elements. Two terminals, V1 340 and V2 342, are tied to the proper DC voltages to control the current through the VCOM lines 344 and produce heat close to the LC. The DC voltage has no effect on the AC video voltages stored in storage capacitors (Cstg), and therefore there is no effect on the pixel voltage (of pixels 348). With the active layer to shield the LC from the heater voltage, it is possible to heat the display during normal operation without introducing visible image artifacts. The heater terminals V1 340 and V2 342 are preferably externally accessible to the display package so that the heater can be controlled separately from the operation of the display circuits.